add

Add a bond to the molecule. If the second parameter is true, there an exception thrown when there is already a
bond between the given MolAtoms. There is no checking if either is SgroupAtom. If the molecule graph is a
superGraph, the bond is also added to its endpoints (two MolAtom objects).

fuse0

check - true if a containment check should be performed to ensure that only those atoms and
bonds are added that are not already elements - false if the graph is known to be
disjoint from this graph and this check can be safely skipped